It is sometimes required to pass a rotating shaft through the wall of a vessel containing a fluid which, for reasons of hazard or expense cannot be permitted to leak to atmosphere even in small amounts. Since there is, inherently, some leakage with any mechanical seal it is a common practice in such cases to use mechanical seals in pairs while supplying a harmless barrier fluid to the cavity between them. The fluid is maintained at a pressure slightly higher than the pressure of the hazardous or expensive fluid. By this means it is assured that any leakage to atmopshere is only the inexpensive or harmless barrier fluid. To ensure maintenance of adequate pressure and make up for normal leakage of barrier fluid (both to atmosphere and into the vessel) various supply systems are employed. Commonly this involves the use of an electric pump activated by a low pressure sensing switch. Should pressure drop below a preset level the pump will supply fluid from a simple reservoir.
Serious failure of a seal is usually preceded by a sudden increase inleakage rate. With a double seal leakage to atmosphere is readily seen, leakage into the sealed vessel is hidden and might not be detected until the barrier fluid was heavily contaminated with the vessel fluid and therefore escaping to atmosphere with the normal leakage of the outermost seal.